This invention is related to inter-device communications between medical devices and most particularly to systems that employ sub stimulation threshold pulses for such communications.
The high cost and general level of difficulty in communicating with an implanted medical device using a low cost external instrument has prevented widespread usage of the data which is currently available from pacemaker and other implantable medical devices to augment traditional transtelephonic home follow-up.
Health care systems are increasingly emphasizing and rewarding those products which reduce the cost of obtaining, communicating, and managing patient data. Therefore inexpensive devices for remotely monitoring the essential status of pacemaker patients and patients with other implantable medical devices is highly desirable. Even small improvements may have significant economic and medical benefit.
Difficulties arise in transferring large amounts of data between an implanted medical device and external monitors or other medical communications systems. Telemetry using RF or E fields and H fields is commonly practiced in, for example, the field of implantable devices such as pacemakers and defibrillator/cardioversion devices in communicating information between the implant and the external transceiving device for example, a programmer. This has limitations as well, primarily on the cost for the external device which goes up considerably if it needs to receive telemetry. Also, the energy cost of transmitting information from the implanted device to outside the patient""s body is higher than using subthreshold electrical pulses and this therefore depletes the implant""s battery, weighing against using telemetry too. The overriding consideration for employing external devices to receive data through skin contact electrodes is the simplicity and low cost of the one-way (receiving) device. (The receiving device could even be worn like a wrist watch and receive subthreshold communications for later retransmission).
Therefore to enable better device transmitted communications as the data amounts and transfer rates are desirably increased, a communications protocol and implementing hardware that facilitates such communications has been developed and is the subject of this document.
A list of references where similar or related inventions in the same or other unrelated fields were contemplated follows, and is incorporated into this disclosure by this reference thereto.
Additionally the Cardiac Telecom HEARTTrac(tm) cardiac monitoring system may provide additional information about such communications but at this date the inventors have not had an opportunity to review this matter.
There still is a need for a very inexpensive method of getting large amounts of data from an implanted device to an external device that is as yet unsatisfied by this art. This is especially true in rural areas and in places where sophisticated telemetry systems may be difficult to use or obtain.
In general this invention provides a way for an implantable medical device to communicate a limited amount of stored data or sensor or status data such as battery status and lead condition to an inexpensive external instrument. Additionally it would be an advantage to be able to also transmit marker data for electrocardiograms. Rather than relying on the more traditional telemetry communications channel which requires a large amount of support circuitry and so forth, we are using certain subthreshold electrical pulsing capability present in some current implantable medical devices for this purpose. This subthreshold pulsing may be delivered along different pathways for minute ventilation, lead impedance, and capture detection, as well as for this new communications purpose. In a preferred embodiment this circuit 10 outputs pulses at rates up to 125 Hz. By modulating a series of such pulses we can easily send data at 10 to 100 bps or even higher data rates. Preferably, communication occurs on a dedicated set of such pulses.
The pulse train can be by modulated to include data in several ways. The form (its amplitude or width for example) of the wave of the communications pulse may be varied in discrete steps. Including or omitting pulses at a given time in a segment length of time can represent various forms of data. Pairing of pulses to send a data bit may be employed. For example, a zero (0) bit could be represented by a pulse followed by a missing pulse, while a one (1) would be represented by a missing pulse followed by a pulse. By limiting ourselves to having at least one missing pulse every two pulse locations, we eliminate the possibility of a 00 or 11 configuration and enhance reliability in reading and allows for easier synchronization by this limitation too. Again, since it is so much less costly we make the communication be only one way. However, so that the implanted device is not communicating constantly to a turned off or disconnected receiver, it is also preferable to trigger a communications episode or session from external to the implanted device. This can be done with a simple xe2x80x9ctelemetry systemxe2x80x9d or a substitute for one like a magnet and an internal reed switch that is in the implant device circuitry and which when triggered by the presence of the magnet, begins a communications episode. (Of course, if a more sophisticated external device is used this sub threshold communication may run simultaneously with or be triggered by the H or E field telemetry. But the preferred embodiments will use simple triggers like sounds or magnets or externally applied electrical pulses, or a short burst of H or E field signal produced by an inexpensive external trigger device.) More specifically, each pulse is adapted to avoid pacing, or any tissue stimulation, and to avoid or minimize its effect on the lead to tissue interface. The size of the electrical pulse energy is therefore below the threshold required for cardiac or skeletal muscle stimulation. These pulses can be safely applied by a pacemaker electrode in a pattern which makes them easily and reliably detectable and interpretable by a simple external device.
A few modifications to currently known devices for delivering subthreshold pulses allows for delivery of modulated pulses. A simple detection algorithm can be implemented in external receivers which normally read electrograms of the patient by use of skin electrodes. The data read can be translated, error-checked, or otherwise modified to transmit the data to the external device. The external device can store this or transmit it to other devices or employ it directly to display diagnostically useful information or device related information for attending technicians or physicians.
In general then the invention is a communications system for communicating between an implanted medical device and a device external to a living body containing said implanted medical device wherein communications of data from within said implanted medical device to said external device is accomplished by a communications circuit for producing modulated biphasic subthreshold pulses in a pattern of modulations predetermined to represent data and insufficiently energetic to cause a physiologically significant reaction in living body tissue, and wherein said modulated pulses are transmitted across two electrodes electrically connected to said implanted device, said electrodes linkable in an electrical circuit from said communications circuit through tissues of said living body, such that said transmission can be received by an external device through a plurality of electrodes connected to said external device when such external device electrodes are in contact with the surface of said body, and the modulations of said subthreshold pulses will be at least one of the set of modulations comprising (adjustments to timing between delivery of pulses, changing amplitude of pulses, absence of a pulse or pulses in a train of pulses, altered or alternating polarity of pulses, and alterations in pulse width).
It has a medical information device for receiving modulated biphasic subthreshold electrical pulses in a pattern of modulations predetermined to represent data and insufficiently energetic to cause a physiologically significant reaction in living body tissue through electrodes for affixation to a living body surface having a detecting circuit for detecting said subthreshold pulses through said electrodes, comprising an amplifier circuit connected to said electrodes and producing an amplified output signal representing an electrical waveform composed substantially of said modulations of said pulses, and having a detecting circuit output for sending said amplified output signal, a decoding circuit comprising a circuit for reading each pulse modulation in said representation of the electrical waveform sent on said detecting circuit output, and for determining a data bit pattern representing data decoded from said modulations in said representation of said electrical waveform, and a conversion circuit for producing a signal representative of the useful information in said bit pattern.
In one preferred form, the decoding circuit determines one data bit value based on based on whether a paired sequence of pulses is in a present-then-absent order, and an opposite data bit value based on an absent-then-present order, in another, the decoding circuit determines a data bit value based on the order of the polarity of a biphasic pulse, in yet another, the decoding circuit determines a data bit value based on whether a biphasic pulse is relatively wide or narrow, and in still another form, the decoding circuit determines a data bit value based on a measure of relative amplitude of a biphasic In fact, the decoding circuit could determine data bit values based on a combination of modulations in said subthreshold pulses.
The useful information communicated can represent marker channel information, data representing physiologic data about a patient or information about a device sending the subthreshold communications from within a body.
The system operates via a method for communicating between an implantable medical device and an external device, starting with some data within an implantable medical device, sending a triggering signal to an implantable medical device, activating said implantable medical device in response to said triggering signal so as to encode and send a modulated set of subthreshold electrical pulses from said implantable device in accord with a protocol having for each data packet a header followed by substantive information, receiving the subthreshold pulses across a pair of electrodes on the surface of the body and decoding modulations of said subthreshold pulses so as to produce a data output representative of the data transmitted by the implantable medical device.
Preferably, the encoding further adds in error correcting code data to the modulated subthreshold pulses in each packet.
On the other side of the communications system is the implantable medical device which has a memory for storing data to be transmitted to an external device and a communication circuit for transmitting subthreshold signals representing data stored in said memory across electrodes external to but electrically connected to the communications circuit, wherein said communications circuit has a generating circuit for producing a biphasic pulse having a modulatable characteristic, said producing circuit adapted to configure each biphasic communications pulse in a pulse train in accord with a value represented by a modulation information signal, a conversion circuit for providing to said generating circuit said modulation information signal to control the modulation of said biphasic pulses, and a configuration circuit for translating data signal values from said memory into modulation signal values for sending said modulation values to said conversion circuit in a stream of values representative of an encoded translation of said data values in said memory. It should also have a trigger circuit for receiving a trigger signal from outside a body and for producing an internal trigger signal on such an occurrence, and an initiation circuit to receive said internal trigger signal from said trigger circuit and on such receipt to initiate program control of functioning of said generation, translation, and configuration circuits so as to send a stream of translated, converted and modulated biphasic communications pulses across said electrodes. In one embodiment, the present invention is a system that includes an implantable medical device having a can with surface electrodes positioned for contact with patient tissue. The system also includes a pair of stimulation electrodes for connection to patient tissue and a pulse generation circuit inside the can. The system further includes an electrode switching circuit that is coupled to the pulse generation circuit and delivers electrical stimulation pulses produced by the pulse generation circuit that are above a patient tissue stimulation threshold to the pair of stimulation electrodes as therapy to a patient. The electrode switching circuit also delivers subthreshold pulses produced by the pulse generation circuit to the can surface electrodes in a predetermined pattern of modulations constituting an encoded data signal that propagates as a signal transmission through the patient tissue. The system also includes a control circuit that is coupled to the pulse generation circuit and the electrode switching circuit, causes the pulse generation circuit to selectively generate the stimulation pulses and the subthreshold pulses, and causes the electrode switching circuit to selectively apply the selectively generated pulses to the pair of stimulation electrodes and the can surface electrodes. The system also includes a plurality of electrodes adapted to be electrically connected to a patient""s skin to receive the subthreshold pulses transmitted through the patient tissue. In addition, the system includes an external device coupled to the skin electrodes to detect the encoded data signal.
Of course, any communications to the external device could be done so as to later be sent by the external device across a telephone or other communications network to a medical information group located at a distant receiver.